1. Field of the Invention
The present invention relates generally to packaging of electronic devices for protection from external environmental conditions. More particularly, the present invention relates to moisture-resistant packages with transparent covers for enclosing solid-state image sensors or similar electronic devices that are sensitive to light or other forms of radiation.
2. State of the Art
Solid-state image sensors, for example, charge coupled devices (CCDs) or complementary metal-oxide semiconductor (CMOS) chips, are increasingly in demand for use with electronic products such as digital cameras. Conventionally, these sensors have been packaged for use by mounting them to a substrate and enclosing them within a housing assembly. The housing assembly incorporates a transparent lid to allow light or other forms of radiation to be received by the sensor. The lid may be a flat window or shaped as a lens to provide optical properties. The substrate and housing are often formed from a ceramic material, and the lid is glass or a similar transparent substance attached to the housing by an adhesive. Due to the materials and structure involved, this packaging technique may be expensive and difficult to manufacture.
In order to overcome these problems, various methods have been developed in attempts to simplify the construction of image sensor packages and reduce material costs. One such approach has been to fabricate packages by using molded housing assemblies that are attached to, or formed on, substrates of low-cost materials like BT/epoxy laminates or plastic. Examples include U.S. Pat. No. 5,811,799 to Wu and U.S. Pat. No. 6,266,197 to Glenn et al. Copending U.S. patent application Ser. No. 10/164,077, filed Jun. 4, 2002, and Ser. No. 10/228,411, filed Aug. 26, 2002, assigned to the assignee of the present invention, also disclose image sensor packages of this type.
While these and other designs have been beneficial with respect to reducing the cost of manufacturing, they raise other concerns. Image sensor packages constructed from materials such as those described above may not provide the same hermetic sealing capabilities as prior art ceramic packaging. Glass laminate-type substrates, for instance, are not completely impermeable and may absorb moisture, especially at edge locations where minute gaps between laminate layers are exposed to the outside environment. This moisture may ultimately find its way into the interior of the package, resulting in damage to the image sensor circuitry. Moisture absorption may also cause deformations in the substrate that will negatively affect the focal properties of the package. Another problem arises from the use of a molded housing, which does not have the high dimensional tolerances of ceramics and may cause difficulties with sealing to the substrate and transparent lid.
The increasing number of portable electronic products intended for use in extreme environments makes reliable sealing of image sensors an even more important aspect of any packaging structure. What is needed is an image sensor package that is simple and inexpensive to manufacture, and that also provides reliable hermetic sealing capabilities to prevent moisture damage and other harmful environmental effects.